gunnera manicata

Hi all, i have a gunnera manicata growing in well aerated and beefed out clay soil, the drainage is good, but although i have had the plant in a year and a half, it has never gotten really big, it is south facing but is shaded after about 4pm in high summer and hasnt been allowed to dry out, the soil is kept decently moist but not boggy, each year it starts off well but then seems to die back....i had been using dilute chicken manure and this year it started well again and i used this water with a small bit of chicken manure and again i noticed the leaves dying back.....so i checked it out and again the crown was rotting a bit with lots of small flies crawling around it, there were 4 decent offshoots with good root surrounding the main dying leaves which i teased off and replanted well away from the original...apart from watching each new plant in different habitats to see which suits, aspect etc and not using the chicken manure, i hope someone can advise me in the care of this plant, as the crown has survived over two severe winters (as low as minus 20) so it is seriously hardy!!